This attribute selects the routines which are used to draw graphics by
the Plot class. If it is zero, then the routines in the graphics
interface selected at link-time are used (see the ast_link script).
Otherwise, routines registered using AST_GRFSET are used. In this
case, if a routine is needed which has not been registered,
then the routine in the graphics interface selected at link-time is
used.
The default is to use the graphics interface